In ink development work with a new technology, one of the most important parameters to be considered is the bleed control between black and color inks, often referred to as "K/C control" or "K/C bleed". Using self-dispersed pigments as the colorant tends to result in relatively poor bleed control, compared to pigments that use a separate dispersant molecule. In these inks, the black ink contains the pigment, while the color inks contain one or more appropriate water-soluble dyes (cyan, yellow, magenta).
One might ask why it is so difficult to control bleed with self-dispersed pigments even though they have a negative charge and the color ink is low pH. The answer to this question is crucial in inventing a solvent system that can reduce bleed. Self-dispersed pigments, such as available from Cabot Corp., have negatively-charged groups that are covalently-bonded to the pigment surface. Theoretically, either a positive charge from cations, or a proton from a low pH color ink in the vicinity of two negatively-charged black, self-dispersed pigment particles should lower their electrostatic potential and allow the pigments to come closer and agglomerate. This apparently does not happen to the desired extent with the self-dispersed pigment and they don't crash with the same intensity on the paper. Without subscribing to any particular theory, it appears that their mutual collisions do not lead to agglomeration; in other words, their collisions are elastic in nature in that the two colliding particles bounce back away from each other. This is in contrast to a pigment-based ink using a separate dispersant, such as disclosed in U.S. Pat. No. 5,500,082, issued to H. Matrick et al. Such dispersant molecules with large "tentacles" (long chain carbons) sticking out and which have a more chance to "stick together" in the event of a collision.
Thus, there is a need to provide an ink containing a self-dispersing pigment that evidences improved bleed control.